The retina is the innermost layer of an eyeball and is the place where optical images created by the lens of the eye is focused. The information from the images are turned into nerve impulses, which are then sent to the brain via the optic nerve. If the retina does not coincide with the resultant focal point of the optical elements of the eye, defocus is generated. As used herein, the term “defocus” refers to the shift of the optical images to a point behind or in front of the retina. The human eye has a feedback mechanism that regulates the growth of the eye to achieve an optimal balance between the size/length of the eye and the focal length of the optical elements of the eye. This feedback mechanism is called emmetropization.
Myopia and hyperopia are common refractive disorders of human eyes. They are generally described as an imbalance between the focusing power of optical elements of the eye and the size/length of the eye. Focus of a myopic eye lies in front of the retina of the eye, while focus of a hyperopic eye lies behind the retina of the eye. It is generally accepted that these disorders are results of inaccurate axial growth during postnatal development of the eyes. In other words, myopia typically develops when the size/length of the eye grows to exceed the focal length of the optical elements of the eye, while hyperopia typically develops when the size/length of the eye grows to be shorter than the focal length of the optical elements of the eye.
Referring to FIG. 1, an optical image 12 is formed in front of the retina in the case of myopia. Defocus in this case is positive and called myopic defocus 13. The emmetropization mechanism operates to retard eye growth in size until the retina 11 coincides with the optical image 12 when the myopic defocus 13 diminished. As a result, the eye becomes less myopic.
Referring to FIG. 2, optical image 22 is formed behind the retina 21 in the case of hyperopia. Defocus in this form is negative and called hyperopic defocus 23. The emmetropization mechanism operates to promote eye growth in size until the retina 21 coincides with the optical image 22 when the hyperopic defocus 23 diminished. As a result, the eye becomes less hyperopic.
Referring to FIG. 3, the natural major sources of defocus for a human eye come from accommodation lag and ambient defocus. The accommodation lag is generally projected by the object of interest 35 onto the center of the retina 31 or macula 34 along a visual axis 32. It usually ranges from 0.5 D to 1.0 D of hyperopic defocus 36 for a non-presbyope during near visual tasks, such as reading. Ambient defocus is projected by peripheral visual objects other than the object of interest 35. Since peripheral objects are usually positioned more distant than the object of interest 35, they usually produce myopic defocus up to 3.0 D during near visual tasks. For example, peripheral object 37 produce myopic defocus 38 at periphery of retina 31. Habitually, the peripheral visual objects are seldom positioned closer than the object of interest 35. However, if they do like peripheral object 39, hyperopic defocus 33 will be produced.
The natural process of emmetropization is regulated by the equilibrium between the above opposite defocus. Incidences of refractive errors are secondary to the disruption of the equilibrium. For example, insufficient ambient myopic defocus may cause myopia. On the other hand, excessive ambient myopic defocus may cause hyperopia.
Existing optical aids and refractive surgeries, in the form of spectacles, contact lens, corneal implant or shape modification of cornea, are corrective approaches involving alteration of the gross focusing power of the eye to produce sharper retinal images. They do not eliminate or deal with the cause of the disorders, but are just prosthetic.
The existing optical treatments to retard the progression of myopia by relieving the eye's accommodation during near visual tasks are recently shown to be clinically ineffective. Examples of those treatments include bi-focal addition lenses, multi-focal progressive addition lenses and their derivatives, and spherical aberration manipulations.